I think you'll be pleasantly surpised, especially if it's warm enough to get wet when you visit.
At UNIVERSAL there is an area called Woody Woodpecker Kidzone. It contains, ET-the ride, Animal planet show, Barney show, Barney's backyard (a GREAT indoor educational interative play area for young kids, go while a barney show is in progress and it will be empty), Curious George wet play area, Dry ball play area and Fievel's water slide(your 3 1/2 yr old might be too young for this but there some interactive play things nearby).
Also, there is an attraction called the Hanna-Barberra ride. You may have to sit on a stationary seat with the youngest but the 6 yr old can sit in the moving seats.
There's is Nickleodean (sp?) studios but I've heard mixed reviews on that. My 5 year old rode Jaws but probably wouldn't want to go again.
At ISLAND OF ADVENTURE there is:
All of Seuss Land with it's rides and street show,
Lost Continent has the talking fountain (just a regular fountain but it's very humorous), The Sinbad show: very funny corny stunt type show;has some loud bangs.
Jurrassic Park has the Unicorn Rollercoaster (bigger than a kiddie coaster but still small, not sure of min heighth), Discovery Center, Triceratops encounter and Camp discovery (a play area with caves and bridges, cool). Also in that area is a ride call (SP?) Pteranadon flyers but it is not worth a long wait, because the ride itself is so short.
Toon Lagoon: Me ship, the Olive is a interactive play area and this whole area has some eye popping colors most kids really enjoy. I believe this area has a stadium show and defenitely a toon trolley street show. the characters sing and dance and sign autographs or pose for pictures.
Marvel Super Hero: There are characters on the street (on and off) like Spiderman,Dr Doom, Captain America, etc. There is a tea cup like ride here call Storm Force.
I'm sure there are other things that I've missed. Baby swap is available on all the big rides too. Don't miss Spiderman-the ride!! If you think you might use that option you might want to think of bringing the 2way radios. They worked great for us.
Good luck!